7.3 Order of Draw and Tube Additives
Key Takeaways
- CLSI-published order of draw is blood culture, sodium citrate (light blue), serum tubes (red/gold/speckled), heparin (green), EDTA (lavender/pearl/pink), then sodium fluoride/potassium oxalate (gray).
- SPS/sterile culture containers come first; citrate requires a 9:1 blood-to-anticoagulant fill; clot activator and gel produce serum; heparin inhibits thrombin; EDTA (K2 or K3) chelates calcium; fluoride/oxalate inhibits glycolysis.
- EDTA carryover falsely lowers calcium and can raise potassium; EDTA into a citrate tube invalidates PT/aPTT; gel can contaminate trace-metal collections.
- A short-draw light-blue citrate tube invalidates coagulation tests because the 9:1 ratio is lost.
- Mix by gentle inversion using typical manufacturer IFU counts (citrate about 3–4; EDTA and heparin about 8–10), not a secret AMT number; shaking hemolyzes cells.
CMLA II.3.B.8.a requires observing proper order of draw. Chapter 6 explained what serum, plasma, EDTA, citrate, heparin, and fluoride/oxalate are. This section is the sequence that keeps one tube's additive from ruining the next. The current venous-collection standard is CLSI PRE02, 8th edition (2025). Facility color charts that contradict PRE02 without a documented reason are not the exam answer.
Quick Answer: Blood culture → light-blue citrate → serum (red/gold/speckled) → heparin green → EDTA lavender/pearl/pink → gray fluoride/oxalate. Order exists to stop carryover. Light-blue short draw invalidates coagulation (9:1). Invert by typical IFU counts; do not shake.
Why order exists
The same needle carries a film of additive from one tube into the next. Order of draw sequences tubes so the most damaging carryover is least likely. It applies to evacuated-tube holders, butterflies, and syringe-plus-transfer-device fills. Drawing only two tubes does not make order optional if those two are citrate and EDTA in the wrong sequence—or EDTA before a calcium/potassium chemistry tube.
CLSI-published order (PRE02 / CLSI insights):
- Blood culture tube or bottle
- Sodium citrate (light blue)
- Serum tubes, including clot activator and gels (red, gold, speckled)
- Heparin with or without gel (green)
- EDTA with or without gel (lavender, pearl, pink)
- Sodium fluoride/potassium oxalate glycolytic inhibitor (gray)
Unlisted tubes—acid-citrate-dextrose, thrombin, royal-blue trace-element, black sedimentation-rate—are placed by additive-carryover risk, not by leftover supply in the basket. A royal-blue clot tube travels with serum tubes; a royal-blue EDTA tube travels with EDTA. Gel next to a trace-metal tube is a contamination risk.
Additive function by position
| Order | Typical closure | Additive and job | CMLA integrity point |
|---|---|---|---|
| 1 | Culture bottle or yellow SPS tube | Sodium polyanethol sulfonate (SPS) and sterile broth; SPS is an anticoagulant that also inhibits complement so organisms can grow | Sterility first: never fill a culture after EDTA or other additive tubes |
| 2 | Light blue | Sodium citrate binds calcium at a calibrated 9:1 blood-to-anticoagulant ratio for PT, aPTT, INR | Underfill over-anticoagulates and invalidates coagulation |
| 3 | Red, gold, red-speckled | Clot activator (silica) ± separator gel for serum chemistry and serology | Gel is not an anticoagulant; the tube must still clot |
| 4 | Green, light green | Heparin (lithium or sodium) inhibits thrombin for plasma chemistry; gel = plasma separator tube | Heparin carryover prolongs coagulation tests |
| 5 | Lavender, pearl, pink | EDTA (K2 spray-dried or K3 liquid) chelates calcium; lavender CBC, pink blood bank, pearl gel plasma-preparation tube | EDTA carryover drops calcium and can raise potassium |
| 6 | Gray | Sodium fluoride (glycolytic inhibitor / preservative) plus potassium oxalate (anticoagulant) | Fluoride preserves glucose; oxalate is the anticoagulant |
K2 EDTA is spray-dried on the tube wall in many current products; K3 EDTA is often liquid and slightly dilutes the specimen. Both chelate calcium. Neither is a coagulation tube.
Carryover traps
EDTA into a chemistry tube. Chelation falsely lowers calcium (and can affect magnesium). K-EDTA falsely raises potassium. That is why EDTA is after heparin and serum, not before them.
EDTA into citrate. Extra chelator and wrong additive make PT/aPTT uninterpretable. Citrate must be collected before EDTA.
Heparin into citrate. Thrombin is inhibited in the coagulation specimen, so times prolong.
Clot activator or serum into citrate. Activator can trigger clotting and ruin coagulation.
Gel into trace metals. Separator gel and some stopper materials contaminate trace-element assays. Place unlisted royal-blue tubes by their own additive, away from gel carryover.
Nonsterile additive into a culture. SPS/sterile containers are first so the needle is not wet with EDTA, heparin, or gel when it enters the bottle.
Light-blue short draw
A short-draw light-blue tube does not contain nine parts blood to one part citrate. Excess citrate binds the calcium the assay adds back, so clotting times prolong artifactually. The specimen is invalid—not a little off. Do not add blood from another tube, do not pour EDTA into it, and do not top off with saline. Recollect a full tube. Butterfly tubing that is not cleared with a discard tube when citrate is first is a classic underfill.
Short draws in other additives are Section 7.4. CMLA highlights citrate here because the ratio is the test.
Mixing: typical IFU, not a secret AMT number
Anticoagulant and clot activator sit in the tube before blood arrives. Invert—gentle complete turns—immediately after fill. Do not shake (hemolysis, Section 7.4).
Typical manufacturer instructions for use (IFU) teach about 3–4 inversions for citrate, about 5 for clot-activator serum tubes, and about 8–10 for heparin, EDTA, and fluoride/oxalate. Blood-culture bottles follow the bottle IFU (often a similar 8–10 range). Those counts are typical IFU teaching, not a hidden AMT exam number. If the tube label differs, the IFU wins.
In practice
The order is blood cultures, CBC, basic metabolic panel (serum or heparin plasma per directory), PT, and glucose in fluoride. You draw: culture bottles, light-blue citrate filled to the line, gold or green chemistry tube, lavender EDTA, gray. You invert each additive tube the IFU count as it comes off the needle. You do not draw the lavender first because the nurse said the CBC was the priority, and you do not pour leftover EDTA into the blue top when it underfills.
Exam traps
- Reciting an old mnemonic that puts serum last or EDTA first.
- Thinking order is optional on a syringe or butterfly.
- EDTA before calcium/potassium chemistry or before citrate.
- Treating a short-draw light blue as valid if you mix extra hard.
- Calling inversion counts a secret AMT number instead of typical IFU.
- Assuming gel is an anticoagulant or that SPS is just another EDTA.
When a stem names two stopper colors, answer with carryover chemistry—calcium, potassium, PT—not with which tube was closer in the tray.
Which sequence matches the CLSI-published order of draw?
EDTA carryover into a subsequent tube most characteristically causes which problem?
A light-blue citrate tube is underfilled, and an EDTA tube is shaken instead of inverted. Which statement is correct?